Heat Treatment Effects on Strawberry Plant Survival and Angular Leaf Spot, Caused by <italic>Xanthomonas fragariae</italic>, in Nursery Production

Angular leaf spot (ALS), caused by the bacterium Xanthomonas fragariae, is one of the most damaging diseases in strawberry nursery production (17). The pathogen was first reported in Minnesota in 1962 and has since been found in several U.S. states as well as strawberry-growing regions in Europe, South America, Africa, Australia, and New Zealand (12,17). The disease affects the foliage, often attacks the calyx, and can move systemically within the vascular system of the plant to infect additional leaf tissue, crown tissue, and developing daughter plants (10,19). Plants with infected crowns are less productive and may die if infection is severe. Also, systemically infected plants likely produce the first infected leaves and serve as the primary source of inoculum in newly planted fields. Secondary cycles of infection result from bacteria exuded from foliar lesions and disseminated by splashing and wind-driven rain or irrigation water. Some commercial cultivars can tolerate foliar infections because the direct impact on yield is thought to be minimal. Infection of fruit calyces, however, leads to a symptom known as “black cap” that reduces the quality and marketability of the fruit. In some commercially important cultivars, however, severe foliar infection stunts growth and reduces the plant canopy, leaving the fruit susceptible to sun scald. The pathogen is transmitted to production fields almost exclusively through infected nursery stock. This creates problems for U.S. nurseries that export plants to Europe, because the European and Mediterranean Plant Protection Organization (EPPO) lists X. fragariae as an A2 quarantine pathogen (i.e., a pathogen absent from the majority of the strawberry-growing countries in Europe, but with the potential to become established there; 25), as well as to U.S. growers who expect to receive clean plants. Nurseries wishing to export plants to some European countries must maintain specific phytosanitary standards. Planting material must be derived from mother plants certified free of X. fragariae and production sites should be documented free from ALS for the past five growing seasons (25). In situations where EPPO’s ALS standards were not met, it would be beneficial to have available a postharvest procedure capable of killing X. fragariae. One possible method is hot-water or heat treatment (15,20,27,29). Heat treatment (HT) has been shown to be effective against systemic pathogens and has been used to reduce or eliminate systemic bacterial infections in propagation material in crops such as apple (3), cherry (6), and grape (11). In strawberry, hot-water treatment for 30 min at 37.8°C is currently being used by some nursery, government, and university breeding programs to eliminate cyclamen mite (4). Buchner (2) found strawberry plants to be intolerant to temperatures exceeding 51 to 52°C for 7 min. Less severe HT delayed growth for up to 2 to 3 weeks. These adverse effects make HT unsuitable for plants destined for fruit production fields but they have a lesser impact in nursery production and would likely be acceptable if the treatment could substantially reduce the pathogen or disease in the early stage of production. In the end, Buchner (2) recommended 5to 7-min dips at 48 to 49°C for control of some insects and diseases in California nursery production; unfortunately, this treatment is insufficient for killing X. fragariae (9). Where hot-water treatment is used (e.g., to eliminate cyclamen mite), the current ABSTRACT Turechek, W. W., and Peres, N. A. 2009. Heat treatment effects on strawberry plant survival and angular leaf spot, caused by Xanthomonas fragariae, in nursery production. Plant Dis. 93:299308.